1. Field of the Invention
The present invention relates to a directional coupler (hereinafter simply referred to as a “coupler”) that picks up part of an output signal, and particularly relates to a coupler formed by a thin-film formation process which is advantageous for achieving thinner products with smaller sizes.
2. Description of Related Art
Radio communication devices are constituted by various kinds of high-frequency components such as antennas, filters, RF switches, power amplifiers, couplers, baluns, etc. In general, a coupler is used for the purpose of picking up part of an output of a power amplifier and feeding it back to an input to the power amplifier so as to maintain and control a constant output gain of the power amplifier.
In recent years, there has been a demand for further thinner and smaller couplers to be used in mobile communication devices such as cellular phones and portable terminals as well as wireless LAN devices. For couplers responding to that demand, a stacked coupler is known, the stacked coupler having a layer of a main line and a layer of a secondary line arranged via an insulating layer, and electromagnetic coupling is generated between the layers (see Patent Document 1). In the stacked coupler, a plurality of vias is formed to ensure electric conductivity between the layers. In the coupler disclosed in Patent Document 1, the main line and the secondary line are each wired out to the opposite sides of the insulating layer through the vias.
Patent document 1: Japanese Unexamined Patent Publication No. 2003-069316